1. Field of the Invention
The invention relates generally to transportation safety equipment and instruments and specifically to devices capable of detecting and measuring liquid water and/or ice accumulation layer, such as can occur on the surfaces of airplane wings and space craft prior to launch.
2. Description of the Prior Art
Ice build-up on low temperature fuel tanks, airfoil surfaces and highways can unexpectedly occur and create hazardous conditions for transportation.
Any amount of ice or slush is considered a contaminant on a wing and presents a potential hazard, therefore the regulations of the Federal Aviation Administration outlined the "clean wing" concept: before an aircraft takes off, there can be no ice present on the airfoil (the wing and upright tail assembly). The "clean wing" concept will have been achieved when (under icing conditions) an aircraft has been properly deiced and, if freezing precipitation is occurring, anti-iced with a fluid having an adequate holdover time interval between the start of anti-icing and the start of takeoff.
Despite this precise regulation, incidents regularly occur that indicate that pilots may not always be able to readily detect the ice buildup and may take off with ice still adhering to the surface of the wing or tail.
While various methods have been tested, currently no product has been approved by the FAA that can measure whether there is ice on the airfoil, discriminate that from other fluids that might be on the wing such as deicer or slush or frost or water, and report to the pilot the relative safety of the aircraft. Various techniques including ultrasonic, lasers, fluorescent dyes and vibrating probes have been tried unsuccessfully. The natural water layer on top of the ice has been a major obstacle in finding a solution. Currently ice detection is done in a low-tech, manual fashion: visually by the pilot and/or one or more members of the ground crew. Under normal winter procedures the top of the wing is inspected for ice when the ambient temp is below 32.degree. F. As the temperature and other climatic conditions change, varying degrees of inspection are required. While the visual inspection is the main method of detection, at least a dozen interrelated variables actually contribute to the detection of ice on the airfoil: ambient temperature, aircraft surface temperature, relative humidity, solar radiation, wind velocity and direction, presence of deicing fluid, type of deicing fluid and its strength, the deicing procedure used, proximity to other aircraft, equipment and buildings, and the aircraft component inclination. All of these combine in varying degrees to make the situation for ice to form.
Ice build-up, such as on the low temperature fuel tanks of space shuttles, is also a safety concern. After filling the insulated fuel tanks on a booster rocket, the countdown time period is allowed to continue unless any ice build-up is one-quarter inch thick or more. Presently, ice depth measurements are done manually by workers who scratch away at an ice layer and measure the ice layer thickness.
Commercial airline disasters in Washington D.C., Denver, Colo., Newfoundland, and recently in Europe, have been suspected to have been caused by ice and snow build-up on the wings of the aircraft. In a preventative attempt to alleviate the potential dangers of ice and snow, air maintenance crews universally spray air foil surfaces with a deicing and anti-icing liquid, e.g., ethylene-glycol, at times not caring whether any ice is actually present. In bad weather operating conditions, takeoffs of planes are often delayed because the weather reduces the number of planes that can takeoff and land, which only exacerbates the icing problem because more time is available for the ice to build-up on wings to dangerous thicknesses. This reduces the hold time of the aircraft before takeoff. Hot ethylene-glycol fluid may be sprayed on airfoil surfaces for deicing purposes. Anti-icing ethylene-glycol fluids are sprayed on airfoil surfaces to create a layer for clearing off a wing during takeoff. As snow or rain continues to accumulate, the freezing-point temperature of the anti-freeze mixture increases. During a taxi and hold period, the effectiveness of the anti-icing fluid is compromised. A pilot's vision of his aircraft's surfaces is usually very limited. Pilots waiting to takeoff need a reliable sensor technology that can determine if critical airfoil surfaces have been compromised. Any ice thickness, snow thickness, and slush-ethylene glycol mixture thickness are all important data a pilot would want to have reported. The freezing point of the anti-icing layer must be known.
General aviation operating in icing conditions typically employ deicing technology to remove dangerous ice formations from air foil surfaces. Ice has a tendency to form on the leading edges and other protruding surfaces of an aircraft's superstructure during flight. Through the years, deicing technology has been developed that includes pneumatic bladders, heating elements, and ultrasonic transducers. Such technology requires in-flight fuel during operation.
Many prior art technologies have been investigated and dismissed as being unreliable in their ability to measure ice build-up under adverse weather conditions. One of the technical problems relates to the discrimination of ice, snow, and water conditions on the surface. Another relates to the measurement of the freezing-point of an anti-freeze mixture. Measurement of the overlying material thickness is another problem. The measurement depends on the electrical parameters of the particular layer and a method of measuring the electrical parameters of the layer is needed. The same technology is needed to determine the freezing-point of an anti-freeze mixture. Sensor wear is yet another problem. For example, sensors that protrude may unreliably determine ice conditions on airfoil surfaces. A flush sensor that can be conformably mounted is needed. Such a sensor must also be compatible with the thermodynamic properties of surrounding surfaces that are to be monitored by the sensor.
Theoretical and experimental studies of microstrip antennas have shown that an antenna's terminal admittance can be made to be dependent on the depth and dielectric constant and electrical conductivity of an ice, snow, water, water-ethylene glycol or coal layer overlying the antenna. For background art, see U.S. Pat. No. 5,072,172, issued Dec. 10, 1991, and especially the discussion relating to FIG. 10, an airplane wing cross-section. Microstrip antennas are typically constructed by forming conductive layers on a substrate with a relative dielectric constant ( .sub.r) greater than 2.2. The requisite physical size of an antenna will decrease with increases in the dielectric constant. Typical values of relative dielectric constant vary from 2.2 for DUROID.TM., to approximately 9.8 for TMM-10.TM. substrate material, which has a lower temperature coefficient. Higher dielectric constant substrates are also technically possible. A microstrip patch may be circular, rectangular or spiral in form. The spiral type may be considered to be a narrow rectangular line with radiation occurring along the edges of a microstrip line. The spiral form may be more sensitive to ice thickness.
Today, there is no working device that is approved for airline use that can measure the presence of ice, slush, snow or water on an aircraft wing, and tell the pilot whether it is safe to take off.
Ice buildup on the massive space shuttle's external tank has always been a safety concern prior to launch into orbit. Ice sloughing-off from the low temperature fuel tanks during launch can damage windows and heat shield tiles on the space shuttle, potentially causing a catastrophic emergency, either at liftoff or on reentry. Therefore, launch criteria requires that there must be no ice formations in an area that could damage windows, and there must be no ice acreage with thickness greater than one-sixteenth inch that could break loose and strike the space shuttle's heat shield tiles and thereby compromise its ability to safely return to earth.
Despite the extraordinary technology developments in many space projects, sensors that can detect ice on the fuel tanks of the space shuttle have not been developed and this problem still persists. Presently, an "ice team" manually scratches the airfoil surfaces to detect ice and measure its thickness. On the large upright external tank, the number of manual measurements possible is limited by practical constraints.